System and method for capacity control in a multiple compressor chiller system

ABSTRACT

A capacity control algorithm for a multiple compressor liquid chiller system is provided wherein the speed and number of compressors in operation are controlled in order to obtain a leaving liquid temperature setpoint. In response to an increase in the load in the chiller system, the algorithm determines if a compressor should be started and adjusts the operating speed of all operating compressors when an additional compressor is started. In response to a decrease in the load in the chiller system with multiple compressors operating, the algorithm determines if a compressor should be de-energized and adjusts the operating speed of all remaining operating compressors when a compressor is de-energized.

BACKGROUND OF THE INVENTION

The present invention relates generally to controlling the capacity of achiller system. More specifically, the present invention relates tocontrolling the capacity of a multiple compressor chiller system havinga single variable speed drive to power the compressors of the chillersystem.

Many liquid chiller or refrigeration applications use multiplecompressors, i.e., two or more compressors, in one or more correspondingrefrigerant circuits. One purpose for the use of multiple compressors isto obtain an increased capacity from the chiller system, which increasedcapacity could not be obtained by operating a single compressor. Inaddition, the use of multiple compressors can provide for improvedreliability of the overall system by having one or more compressorsremain operational to provide a reduced level of cooling capacity in theevent that a compressor fails and can no longer provide coolingcapacity.

The compressor motors of the chiller system can be powered directly fromthe AC power grid at the system location, which would result in thecompressor being operated at only a single speed. Alternatively, thecompressor motors can use a variable speed drive inserted between thesystem power grid and the motor to provide the motor with power at avariable frequency and variable voltage, which then results in thecompressor being capable of operation at several different speeds.Variable speed operation of the motors can be obtained by providing acorresponding variable speed drive for each compressor motor or byconnecting all of the compressor motors in parallel to the inverteroutput of a variable speed drive. One drawback of using a variable speeddrive for each compressor is that the overall chiller system becomesmore expensive because multiple drives with a given cumulative powerrating are more expensive than a single drive of the same output powerrating. One drawback to connecting the compressor motors in parallel tothe single inverter output of the variable speed drive is that a faultor failure of one of the motors may disable the variable speed drive andthus prevent the other motors connected to the variable speed drive fromoperating the remaining compressors on the chiller system. Thisdisabling of the other motors connected to the variable speed drivedefeats the function of the redundant compressors because all thecompressors are disabled as a result of the disabling of the motors andthe variable speed drive.

The corresponding control for the compressor motor powered by the ACpower grid is relatively simple involving mainly the starting andstopping of the motor. While the corresponding control for the variablespeed drive powered compressor motor is much more complicated andinvolves determining an appropriate speed for each compressor motor (andcompressor) based on system conditions.

One type of control for multiple compressors involves the sequentialengaging and disengaging of compressors to obtain a desired system load.This control process usually involves the starting of one compressor tomeet an increasing system demand and subsequently adding additionalcompressors until the system demand is satisfied. The compressors arethen shutdown or unloaded in a similar manner in response to adecreasing system demand. One example of this type of control is foundin U.S. Pat. No. 6,499,504 (the '504 Patent). The '504 Patent isdirected to a compressor control system that operates in response toboth the system pressure and the volumetric flow rate capacity of thesystem. Specifically, a compressor is loaded or unloaded from thecompressor system after sensing the actual pressure and volumetric flowrate capacity of the compressor system.

Another type of control process for multiple compressors involvesdetermining an operating configuration for a lead compressor based onsystem conditions and then controlling one or more lag compressors usingadditional control instructions to match the output of the leadcompressor. One example of this type of control is found in U.S. Pat.No. 5,343,384 (the '384 Patent). The '384 Patent is directed to acontrol system and method that operates a plurality of compressors atsimilar operating points. A microcontroller continually compares thesystem pressure with the desired pressure and causes correspondingadjustments, either up or down, in first, the position of the inletvalve of a lead compressor, and subsequently, in the position of thebypass valve of a lead compressor, so that these changes can be passedto the remaining compressors in the system by way of the CEM program.

Therefore, what is needed is a system and method for controlling thecapacity of a multiple compressors chiller system by controlling boththe operating speed of the compressors and the number of compressors inoperation to maintain a leaving chilled liquid temperature setpoint inthe chiller system.

SUMMARY OF THE INVENTION

One embodiment of the present invention is directed to a method forcontrolling the capacity of a multiple compressor chiller system. Themethod includes the step of providing a variable speed drive having aplurality of inverters. Each inverter is configured to power acorresponding compressor motor of a multiple compressor chiller system.The method also includes the steps of monitoring at least one operatingcondition of a multiple compressor chiller system, determining whetherto increase output capacity of a multiple compressor chiller system inresponse to the at least one monitored operating condition, andadjusting an operating configuration of the plurality of inverters toincrease the output capacity of a multiple compressor chiller system inresponse to a determination to increase output capacity. The methodfurther includes the steps of determining whether to decrease outputcapacity of a multiple compressor chiller system in response to the atleast one monitored operating condition and adjusting an operatingconfiguration of the plurality of inverters to decrease the outputcapacity of a multiple compressor chiller system in response to adetermination to decrease output capacity.

Another embodiment of the present invention is directed to a multiplecompressor chiller system having a plurality of compressors. Eachcompressor of the plurality of compressors being driven by acorresponding motor and the plurality of compressors are incorporatedinto at least one refrigerant circuit. Each refrigerant circuit includesat least one compressor of the plurality of compressors, a condenserarrangement and an evaporator arrangement connected in a closedrefrigerant loop. The multiple compressor chiller system also has avariable speed drive to power the corresponding motors of the pluralityof compressors. The variable speed drive includes a converter stage, aDC link stage and an inverter stage. The inverter stage having aplurality of inverters each electrically connected in parallel to the DClink stage and each powering a corresponding motor of the plurality ofcompressors. The multiple compressor chiller system further has acontrol panel to control the variable speed drive to generate apreselected system capacity from the plurality of compressors. Thecontrol panel is configured to determine a number of inverters of theplurality of inverters to operate in the variable speed drive and isconfigured to determine an operating frequency for the number ofoperating inverters of the plurality of inverters in the variable speeddrive to generate the preselected system capacity from the plurality ofcompressors.

A further embodiment of the present invention is directed to a capacitycontrol method for a multiple compressor chiller system. The methodincludes the step of providing a variable speed drive having a pluralityof inverters. Each inverter is configured to power a correspondingcompressor motor of a multiple compressor chiller system at apreselected output frequency. The method also includes the steps ofmonitoring at least one operating condition of a multiple compressorchiller system, determining whether to increase capacity in the multiplecompressor chiller system in response to the at least one monitoredoperating condition, and configuring the plurality of inverters togenerate increased capacity in the multiple compressor chiller system inresponse to a determination to increase capacity. The step ofconfiguring the plurality of inverters to generate increased capacityincludes determining whether to enable an additional inverter of theplurality of inverters in order to start an additional compressor motorof the multiple compressor chiller system, enabling an additionalinverter of the plurality of inverters in response to a determination toenable an additional inverter, and adjusting the preselected outputfrequency of each operating inverter of the plurality of inverters. Themethod further includes the steps of determining whether to decreasecapacity in the multiple compressor chiller system in response to the atleast one monitored operating condition and configuring the plurality ofinverters to generate decreased capacity in the multiple compressorchiller system in response to a determination to decrease capacity. Thestep of configuring the plurality of inverters to generate decreasedcapacity includes determining whether to disable an operating inverterof the plurality of inverters in order to stop a compressor motor of themultiple compressor chiller system, disabling an operating inverter ofthe plurality of inverters in response to a determination to disable anoperating inverter, and decreasing the preselected output frequency ofeach operating inverter of the plurality of inverters.

One advantage of the present invention is that compressor cycling isreduced, while providing optimum control of the compressors.

Another advantage of the present invention is that system efficiency isimproved by operating as many compressors a possible to satisfy a givenload condition.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a general application that can be used with thepresent invention.

FIG. 2 illustrates schematically a variable speed drive that can be usedwith the present invention.

FIG. 3 illustrates an embodiment of a refrigeration or chiller systemused with the present invention.

FIG. 4 is a flowchart showing the basic capacity control process of thepresent invention.

FIG. 5 is a flowchart showing a compressor starting control process ofthe present invention.

FIG. 6 is a flowchart showing a system loading control process of thepresent invention.

FIG. 7 is a flowchart showing a system unloading control process of thepresent invention.

Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates generally an application that can be used with thepresent invention. An AC power source 102 supplies a variable speeddrive (VSD) 104, which powers a plurality of motors 106. The motors 106are preferably used to drive corresponding compressors that can be usedin a refrigeration or chiller system. A control panel 110 can be used tocontrol operation of the VSD 104 and can monitor and/or controloperation of the motors 106 and compressors.

The AC power source 102 provides single phase or multi-phase (e.g.,three phase), fixed voltage, and fixed frequency AC power to the VSD 104from an AC power grid or distribution system that is present at a site.The AC power source 102 preferably can supply an AC voltage or linevoltage of 200 V, 230 V, 380 V, 460 V, or 600 V at a line frequency of50 Hz or 60 Hz, to the VSD 104 depending on the corresponding AC powergrid.

The VSD 104 receives AC power having a particular fixed line voltage andfixed line frequency from the AC power source 102 and provides AC powerto each of the motors 106 at desired voltages and desired frequencies,both of which can be varied to satisfy particular requirements.Preferably, the VSD 104 can provide AC power to each of the motors 106that may have higher voltages and frequencies and lower voltages andfrequencies than the rated voltage and frequency of each motor 106. Inanother embodiment, the VSD 104 may again provide higher and lowerfrequencies but only the same or lower voltages than the rated voltageand frequency of each motor 106.

The motors 106 are preferably induction motors that are capable of beingoperated at variable speeds. The induction motors can have any suitablepole arrangement including two poles, four poles or six poles. However,any suitable motor that can be operated at variable speeds can be usedwith the present invention.

FIG. 2 illustrates schematically some of the components in oneembodiment of the VSD 104. The VSD 104 can have three stages: aconverter or rectifier stage 202, a DC link stage 204 and an outputstage having a plurality of inverters 206. The converter 202 convertsthe fixed line frequency, fixed line voltage AC power from the AC powersource 102 into DC power. The converter 202 can be in a rectifierarrangement composed of electronic switches that can only be turned oneither by gating, when using silicon controlled rectifiers, or by beingforward biased, when using diodes. Alternatively, the converter 202 canbe in a converter arrangement composed of electronic switches that canbe gated both on and off, to generate a controlled DC voltage and toshape the input current signal to appear sinusoidal, if so desired. Theconverter arrangement of converter 202 has an additional level offlexibility over the rectifier arrangement, in that the AC power cannotonly be rectified to DC power, but that the DC power level can also becontrolled to a specific value. In one embodiment of the presentinvention, the diodes and silicon controlled rectifiers (SCRs) canprovide the converter 202 with a large current surge capability and alow failure rate. In another embodiment, the converter 202 can utilize adiode or thyristor rectifier coupled to a boost DC/DC converter or apulse width modulated boost rectifier to provide a boosted DC voltage tothe DC link 204 in order to obtain an output voltage from the VSD 104greater than the input voltage of the VSD 104.

The DC link 204 filters the DC power from the converter 202 and providesenergy storage components. The DC link 204 can be composed of capacitorsand inductors, which are passive devices that exhibit high reliabilityrates and very low failure rates. Finally, the inverters 206 areconnected in parallel on the DC link 204 and each inverter 206 convertsthe DC power from the DC link 204 into a variable frequency, variablevoltage AC power for a corresponding motor 106. The inverters 206 arepower modules that can include power transistors or integrated bipolarpower transistor (IGBT) power switches with diodes connected inparallel. Furthermore, it is to be understood that the VSD 104 canincorporate different components from those discussed above and shown inFIG. 2 so long as the inverters 206 of the VSD 104 can provide themotors 106 with appropriate output voltages and frequencies.

For each motor 106 to be powered by the VSD 104, there is acorresponding inverter 206 in the output stage of the VSD 104. Thenumber of motors 106 that can be powered by the VSD 104 is dependentupon the number of inverters 206 that are incorporated into the VSD 104.In a preferred embodiment, there can be either 2 or 3 inverters 206incorporated in the VSD 104 that are connected in parallel to the DClink 204 and used for powering a corresponding motor 106. While it ispreferred for the VSD 104 to have between 2 and 3 inverters 206, it isto be understood that more than 3 inverters 206 can be used so long asthe DC link 204 can provide and maintain the appropriate DC voltage toeach of the inverters 206.

In a preferred embodiment, the inverters 206 are jointly controlled by acontrol system, as discussed in greater detail below, such that eachinverter 206 provides AC power at the same desired voltage and frequencyto corresponding motors based on a common control signal or controlinstruction provided to the inverters 206. The control of the inverters206 can be by the control panel 110 or other suitable control devicethat incorporates the control system.

The VSD 104 can prevent large inrush currents from reaching the motors106 during the startup of the motors 106. In addition, the inverters 206of the VSD 104 can provide the AC power source 102 with power havingabout a unity power factor. Finally, the ability of the VSD 104 toadjust both the input voltage and input frequency received by the motor106 permits a system equipped with VSD 104 to be operated on a varietyof foreign and domestic power grids without having to alter the motors106 for different power sources.

FIG. 3 illustrates generally one embodiment of the present inventionincorporated in a refrigeration system. As shown in FIG. 3, the HVAC,refrigeration or liquid chiller system 300 has two compressorsincorporated in corresponding refrigerant circuits, but it is to beunderstood that the system 300 can have one refrigerant circuit or morethan two refrigerant circuits for providing the desired system load andcan have more than a one compressor for a corresponding refrigerantcircuit. The system 300 includes a first compressor 302, a secondcompressor 303, a condenser arrangement 308, expansion devices, a waterchiller or evaporator arrangement 310 and the control panel 110. Thecontrol panel 110 can include an analog to digital (A/D) converter, amicroprocessor, a non-volatile memory, and an interface board to controloperation of the refrigeration system 300. The control panel 110 canalso be used to control the operation of the VSD 104, the motors 106 andthe compressors 302 and 303. A conventional HVAC, refrigeration orliquid chiller system 300 includes many other features that are notshown in FIG. 3. These features have been purposely omitted to simplifythe drawing for ease of illustration.

The compressors 302 and 303 compress a refrigerant vapor and deliver itto the condenser 308. The compressors 302 and 303 are preferablyconnected in separate refrigeration circuits, i.e., the refrigerantoutput by the compressors 302 and 303 are not mixed and travel inseparate circuits through the system 300 before reentering thecompressors 302 and 303 to begin another cycle. The separaterefrigeration circuits preferably use a single condenser housing 308 anda single evaporator housing 310 for the corresponding heat exchanges.The condenser housing 308 and evaporator housing 310 maintain theseparate refrigerant circuits either through a partition or otherdividing means with the corresponding housing or with separate coilarrangements. In another embodiment of the present invention, therefrigerant output by the compressors 302 and 303 can be combined into asingle refrigerant circuit to travel through the system 300 before beingseparated to reenter the compressors 302 and 303.

The compressors 302 and 303 are preferably screw compressors orcentrifugal compressors, however the compressors can be any suitabletype of compressor including reciprocating compressors, scrollcompressors, rotary compressors or other type of compressor. The outputcapacity of the compressors 302 and 303 can be based on the operatingspeed of the compressors 302 and 303, which operating speed is dependenton the output speed of the motors 106 driven by the inverters 206 of theVSD 104. The refrigerant vapor delivered to the condenser 308 entersinto a heat exchange relationship with a fluid, e.g., air or water, andundergoes a phase change to a refrigerant liquid as a result of the heatexchange relationship with the fluid. The condensed liquid refrigerantfrom condenser 308 flows through corresponding expansion devices to anevaporator 310.

The evaporator 310 can include connections for a supply line and areturn line of a cooling load. A secondary liquid, which is preferablywater, but can be any other suitable secondary liquid, e.g. ethylene,calcium chloride brine or sodium chloride brine, travels into theevaporator 310 via return line and exits the evaporator 310 via supplyline. The liquid refrigerant in the evaporator 310 enters into a heatexchange relationship with the secondary liquid to chill the temperatureof the secondary liquid. The refrigerant liquid in the evaporator 310undergoes a phase change to a refrigerant vapor as a result of the heatexchange relationship with the secondary liquid. The vapor refrigerantin the evaporator 310 then returns to the compressors 302 and 303 tocomplete the cycle. It is to be understood that any suitableconfiguration of condenser 308 and evaporator 310 can be used in thesystem 300, provided that the appropriate phase change of therefrigerant in the condenser 304 and evaporator 306 is obtained.

Preferably, the control panel, microprocessor or controller 110 canprovide control signals to the VSD 104 to control the operation of theVSD 104, and particularly the operation of inverters 206, to provide theoptimal operational setting for the VSD 104. The control panel 110 canincrease or decrease the output voltage and/or frequency of theinverters 206 of the VSD 104, as discussed in detail below, in responseto increasing or decreasing load conditions on the compressors 302 and303 in order to obtain a desired operating speed of the motors 106 and adesired capacity of the compressors 302 and 303.

The control panel 110 executes a control algorithm(s) or software tocontrol operation of the system 100 and to determine and implement anoperating configuration for the inverters 206 of the VSD 104 to controlthe capacity of the compressors 102 and 104 in response to a particularoutput capacity requirement for the system 100. In one embodiment, thecontrol algorithm(s) can be computer programs or software stored in thenon-volatile memory of the control panel 110 and can include a series ofinstructions executable by the microprocessor of the control panel 110.While it is preferred that the control algorithm be embodied in acomputer program(s) and executed by the microprocessor, it is to beunderstood that the control algorithm may be implemented and executedusing digital and/or analog hardware by those skilled in the art. Ifhardware is used to execute the control algorithm, the correspondingconfiguration of the control panel 110 can be changed to incorporate thenecessary components and to remove any components that may no longer berequired.

FIG. 4 illustrates the basic capacity control process of the presentinvention. The process begins by monitoring the current operatingconditions of the compressors and the corresponding chiller system atstep 402. One more sensors or other suitable monitoring devices areplaced in the chiller system to monitor one or more operating conditionsof the chiller system. The sensors provide signals to the control panel110 corresponding to the measured system parameters. The measured systemparameters of the chiller system can correspond to any suitable chillersystem parameter that can be measured such as refrigerant temperature,refrigerant pressure, refrigerant flow, leaving chilled liquidtemperature from the evaporator or any other suitable parameter.

Based on the monitored system conditions obtained in step 402, thecontrol process then determines if an initial system startup is requiredin step 404. An initial system startup involves the starting of one ormore compressors to transition the system from an inactive or shut-downstate to an active or operational state. If an initial system startup isdetermined to be necessary, the control passes to a startup controlprocess shown in FIG. 5 and described in greater detail below. If noinitial system startup is necessary, usually because one or morecompressors have been previously started, the control process moves tostep 406 to determine if system loading or increased system capacity isrequired.

If the control process determines that system loading is required inresponse to a demand for additional system capacity based the monitoredsystem conditions in step 402, the control process proceeds to a systemloading process shown in FIG. 6 and described in greater detail below toincrease the load on the compressors in order to increase the systemcapacity. If system loading is not necessary, the control process movesto step 408 to determine if system unloading or decreased systemcapacity is required.

If the control process determines that system unloading is required inresponse to a decrease in the demand for system capacity based on themonitored system conditions in step 402, the control process proceeds toa system unloading process shown in FIG. 7 and described in greaterdetail below to decrease the load on the compressors in order todecrease the system capacity. If system unloading is not necessary, thecontrol process returns to step 402 and repeats the process.

The basic control process of FIG. 4 preferably uses a fuzzy logiccontrol technique, but can use any suitable control technique fordetermining when to start the compressors of the chiller system, when toincrease the capacity of the chiller system and when to decrease thecapacity of the chiller system. The control processes of FIGS. 5, 6 and7 are preferably directed to the control process for the chiller systemin response to one of the above-determinations being made by the basiccontrol process of FIG. 4.

FIG. 5 illustrates a startup control process for the present invention.The startup control process involves the starting of one or morecompressors to transition the system from an inactive or shut-down stateto an active or operational state. The process begins in step 502 bydetermining if all of the compressors are off, inactive or shut-down. Ifone of the compressors is active or operational in step 502, the processreturns to step 402 of FIG. 4 to further monitor system conditionsbecause the startup process is not required because one or more of thecompressors is operational. Next, after determining that all thecompressors are inactive or off in step 502, i.e., the compressors arenot in operation, the startup control process determines if the leavingchilled liquid temperature (LCHLT) from the evaporator is greater than asetpoint temperature plus a predetermined offset or control range. Thepredetermined offset provides for a control region around the setpointtemperature, i.e., the desired LCHLT, to prevent frequent adjustments tothe chiller system in response to very minor changes in systemconditions.

The predetermined setpoint temperature and the predetermined offset canpreferably be programmable or set by a user, but, it is to be understoodthat the predetermined setpoint temperature and the predetermined offsetcan also be preprogrammed into the system. The predetermined setpointtemperature can range between about 10° F. and about 60° F. depending onthe particular liquid to be chilled in the evaporator. The predeterminedsetpoint temperature is preferably between about 40° F. and about 55° F.when water is to be chilled and is preferably between about 15° F. andabout 55° F. when a glycol mixture is to be chilled. The predeterminedoffset can range between about ±1° F. and about ±5° F. and is preferablybetween about ±1.5° F. and about ±2.5° F.

If the LCHLT is greater than the setpoint temperature plus thepredetermined offset in step 504, then the number of compressors to bestarted is determined in step 506. The number of compressors to startcan be determined by any suitable technique and is usually determined inresponse to particular system features or parameters such as the LCHLTand the rate of change of the LCHLT. If the LCHLT is not greater thanthe setpoint temperature plus the predetermined offset in step 504, thenthe process returns to step 402 of FIG. 4 to further monitor systemconditions. After the number of compressors to start is determined, thecompressors are tested in step 508 to determine if the compressors canbe started or operated. In step 508, the control panel 110 canpreferably determine if the compressors cannot be started or operated orare otherwise inoperable based on internal compressor controls orsignals that prevent the starting of a compressor, (e.g., a “no runpermissive” signal is present, the compressor has been faulted or thecompressor is locked out), or based on other system controls or signalsrelating to problems or restrictions in the system, (e.g., the systemswitch has been turned off, the system has been faulted, the system hasbeen locked out, or the system anti-recycle timer is active). If all thecompressors cannot be started in step 508, the process returns to step402 of FIG. 4 to further monitor system conditions. Once it isdetermined that all the compressors to be started are capable of beingstarted and operated, the compressors are started in step 510 andoperated at a frequency corresponding to the minimum frequency output bythe VSD. The minimum frequency output by the VSD for compressoroperation can range from 15 Hz to 75 Hz and is preferably 40 Hz. It isto be understood that the VSD may be capable of providing a minimumfrequency output that is less than the minimum frequency output requiredfor compressor operation. After the compressors are started in step 510,the process returns to step 402 of FIG. 4 to begin the process again andmonitor system conditions.

FIG. 6 illustrates a system loading control process for the presentinvention. The system loading control process involves either theactivating or starting of one or more compressors in response to anincreased load or demand on the system or the increasing of the outputfrequency from the VSD powering the compressors in order to increase theoutput capacity of the compressors in response to an increased load ordemand on the system. The process begins in step 602 by determining if aload timer or counter has completed its count. In one embodiment of thepresent invention, the load timer is preferably set for 2 seconds.However, any suitable time duration can be used for the load timer. Ifthe load timer has not completed its count, the system will not load anyof the compressors and returns to step 402 of FIG. 4 to further monitorsystem conditions until the load timer is finished or system conditionschange. The load timer is used to give the system adequate time torespond to a prior control instruction that started a new compressor orincreased the output frequency of the VSD powering the compressors andtheir respective motors.

After the load timer has completed its count, the system loading controlprocess then determines if there are any compressors that are notcurrently in operation that are capable of operation in step 604. Ifthere are any compressors that are not currently in operation, then theoutput frequency of the VSD, i.e., the operating frequency of thecompressors, is compared to a stop frequency plus a predetermined offsetfrequency in step 606. The stop frequency is preferably calculated asthe VSD minimum frequency output, as discussed above, multiplied by theratio of the number of operating compressors plus one divided by thenumber of operating compressors. The predetermined offset frequency canrange from between about 0 Hz and about 50 Hz and is preferably betweenabout 5 Hz and about 10 Hz. The comparison of the VSD output frequencyto the stop frequency plus the offset frequency is used to determine ifit would be appropriate to start another compressor. The addition of theoffset frequency to the stop frequency is used to prevent the startingof a compressor by just satisfying the condition for starting acompressor, i.e., being at the stop frequency, and then having to shutoff a compressor in response to a decreased load or demand on thesystem, i.e., a call to unload, because the compressors are operating atthe minimum frequency. The addition of the offset to the stop frequencyis used to have the compressors operating at a frequency above theminimum frequency, after an additional compressor is started, so thereis room to unload the compressors by decreasing the output frequency ofthe VSD before a shutdown of a compressor is required.

After determining that the VSD output frequency is greater than the stopfrequency plus the offset in step 606, another compressor is started andthe VSD is controlled to power the operating compressors at a startfrequency in step 608. The start frequency is preferably calculated asthe VSD output frequency prior to starting the compressor multiplied bythe ratio of the number of operating compressors (including the one tobe started) minus one divided by the number of operating compressors(including the one to be started). Once the compressors are started andaccelerated to the start frequency, the process returns to step 402 ofFIG. 4 to further monitor system conditions.

Referring back to step 604, if all the compressors are currentlyoperating, it is determined in step 610 if the VSD output frequencypowering the compressors is less than the maximum VSD output frequency.The VSD maximum output frequency can range between 120 Hz and 300 Hz andis preferably 200 Hz. However, it is to be understood that the VSD canhave any suitable maximum output frequency. If the VSD output frequencyis equal to the maximum VSD output frequency, then the process returnsto step 402 of FIG. 4 to further monitor system conditions because noadditional capacity can be generated by the system. However, if the VSDoutput frequency is less than the maximum VSD output frequency, then thecompressors and their corresponding refrigerant circuits are checked orevaluated to determine if they are approaching an unload limit in step612. The unload limit is used to prevent damage to the compressors andcorresponding refrigerant circuit by unloading the compressors whencertain predetermined parameters or conditions are present.

If no compressors or corresponding refrigerant circuits are approachingan unload limit, then the VSD is controlled to power the compressors atan increased VSD output frequency equal to the current output frequencyplus a predetermined increment amount in step 616. The predeterminedincrement amount can be between about 0.1 Hz and about 25 Hz and ispreferably between about 0.1 Hz and about 1 Hz. The predeterminedincrement amount can preferably be calculated by a fuzzy logiccontroller or control technique, however, any suitable controller orcontrol technique, e.g., a PID control, can be used. The increased VSDoutput frequency can be increased up to the maximum VSD outputfrequency. Once the compressors are accelerated to the increased VSDoutput frequency, the process returns to step 402 of FIG. 4 to furthermonitor system conditions. Referring back to step 612, if it isdetermined that one or more compressors and corresponding refrigerantcircuits are approaching an unload limit, then a limited load valuebased on information in a load limiting controls table is calculated forthose compressors and corresponding refrigerant circuits in step 614.Next, in step 616, as described in detail above, the process adjusts theVSD output frequency for the compressors, subject to any load limitsfrom step 614, and returns to step 402 of FIG. 4 to further monitorsystem conditions.

FIG. 7 illustrates a system unloading control process for the presentinvention. The system unloading control process involves thedeactivating or shutting down of one or more compressors in response toa reduced load or demand for the system or the decreasing of the outputfrequency from the VSD powering the compressors in order to decrease theoutput capacity of the compressors in response to a decreased load ordemand on the system. The process begins in step 702 by determining ifan unload timer or counter has completed its count. In one embodiment ofthe present invention, the unload timer is preferably set for 2 seconds.However, any suitable time duration can be used for the unload timer. Ifthe unload timer has not completed its count, the system will not unloadany of the compressors and returns to step 402 of FIG. 4 to furthermonitor system conditions until the unload timer is finished or systemconditions change.

The unload timer is used to give the system adequate time to respond toa prior control instruction that stopped an operating compressor ordecreased the output frequency of the VSD powering the compressors andtheir respective motors. After the unload timer has completed its count,the compressor unloading control process then determines if only asingle compressor or the lead compressor is currently in operation instep 704. If only a single compressor or the lead compressor is inoperation, then the output frequency of the VSD is compared to theminimum VSD frequency to determine if the output frequency of the VSD isgreater than the minimum VSD frequency in step 706. If the outputfrequency of the VSD is not greater than the minimum VSD frequency, thenthe LCHLT is evaluated to determine if it is less than the setpointtemperature minus the predetermined offset in step 708. If the LCHLT isless than the setpoint temperature minus the predetermined offset instep 708, then the process begins the shut down process for thecompressor and the corresponding refrigeration system in step 710 andthe process ends. The compressor is shut down if the LCHLT is less thanthe setpoint temperature minus the predetermined offset because thesystem has completed its operating objective, i.e., reaching thesetpoint temperature, and, depending on the freezing point of the liquidin the chiller, to possibly avoid damaging the compressor or thecorresponding refrigeration circuit by having too low a LCHLT. If theLCHLT is not less than the setpoint temperature minus the predeterminedoffset in step 708, then the compressor continues operating at theminimum speed and the process returns to step 402 for furthermonitoring.

If the output frequency of the VSD is greater than the minimum VSDfrequency in step 706, then the VSD is controlled to power thecompressor at a decreased VSD output frequency equal to the currentoutput frequency minus a predetermined decrement amount in step 712. Thepredetermined decrement amount can be between about 0.1 Hz and about 25Hz and is preferably between about 0.1 Hz and about 1 Hz. Thepredetermined decrement amount can preferably be calculated by a fuzzylogic control, however, any suitable control, e.g., a PID control, canbe used. The decreased VSD output frequency can be decreased down to theminimum VSD output frequency. Once the compressor is adjusted to thedecreased VSD output frequency, the process returns to step 402 of FIG.4 to further monitor system conditions.

Referring back to step 704, if any of the compressors besides the leadcompressor are in operation, it is determined in step 714 if the VSDoutput frequency powering the compressors is equal to the minimum VSDoutput frequency. If the VSD output frequency is equal to the minimumVSD output frequency, then a lag compressor is stopped or shut down andthe VSD is controlled to power the remaining operating compressors atthe stop frequency in step 716. As discussed above, the stop frequencyis preferably calculated as the VSD minimum frequency output, multipliedby the ratio of the number of operating compressors plus one divided bythe number of operating compressors. Once the remaining compressors arestarted and accelerated to the stop frequency, the process returns tostep 402 of FIG. 4 to further monitor system conditions.

If the VSD output frequency is not equal to the minimum VSD outputfrequency in step 714, then the VSD is controlled to power thecompressors at a decreased VSD output frequency equal to the currentoutput frequency minus a predetermined decrement amount in step 712, asdescribed in greater detail above. Once the compressors are adjusted tothe decreased VSD output frequency, the process returns to step 402 ofFIG. 4 to further monitor system conditions.

While the above control process discussed the controlling of the systemcapacity by adjusting the output frequency of the VSD provided to themotors, it is to be understood that the output voltage of the VSD canalso be adjusted to control the system capacity. In the above controlprocesses, the VSD is preferably controlled to maintain a constantvolts/Hz or constant torque mode of operation. The constant flux orconstant volts/Hz mode of motor operation, which is used for a load witha substantially constant torque profile, such as a screw compressor,requires any increases or decreases in frequency provided to the motorto be matched by corresponding increases and decreases in the voltagesprovided to the motor. For example, a four pole induction motor candeliver twice its rated output horsepower and speed when operated attwice its rated voltage and twice its rated frequency. When in theconstant flux or constant volts/Hz mode, any increase in the voltage tothe motor results in an equivalent increase in the output horsepower ofthe motor. Similarly, any increase in the frequency to the motor resultsin an equivalent increase in the output speed of the motor.

When starting or stopping a compressor in order to adjust the capacityof the chiller system, such as described in steps 608 and 716, the VSDpreferably follows the following procedure. First, the VSD isdecelerated to a zero speed in a controlled stop. Next, the compressorto be added or removed is correspondingly enabled or disabled. The VSDis then controlled to provide output power to the compressors inoperation at either the start frequency, when adding a compressor, orthe stop frequency, when removing a compressor. It being understood thatthe VSD is also controlled to provide the appropriate voltage for thecorresponding frequency. Finally, the VSD is accelerated to theappropriate frequency and voltage to power the compressors in operation.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A method for controlling the capacity of a multiple compressorchiller system, the method comprising the steps of: providing a variablespeed drive having a plurality of inverters, wherein each inverter isconfigured to power a corresponding compressor motor of a multiplecompressor chiller system; monitoring at least one operating conditionof a multiple compressor chiller system; determining whether to increaseoutput capacity of a multiple compressor chiller system in response tothe at least one monitored operating condition; adjusting an operatingconfiguration of the plurality of inverters to increase the outputcapacity of a multiple compressor chiller system in response to adetermination to increase output capacity; determining whether todecrease output capacity of a multiple compressor chiller system inresponse to the at least one monitored operating condition; andadjusting an operating configuration of the plurality of inverters todecrease the output capacity of a multiple compressor chiller system inresponse to a determination to decrease output capacity.
 2. The methodof claim 1 further comprising the steps of: determining whether to starta predetermined number of inverters of the plurality of inverters inresponse to the at least one monitored operating condition; and startingthe predetermined number of inverters of the plurality of inverters inresponse to a determination to start the predetermined number ofcompressors.
 3. The method of claim 2 wherein the step of starting thepredetermined number of inverters of the plurality of inverters includesthe steps of: determining whether the plurality of inverters areinactive; determining whether a chilled liquid temperature is greaterthan a setpoint temperature plus an offset temperature; and determiningthe predetermined number of inverters of the plurality of inverters tostart in response to a determination that the plurality of inverters areinactive and a determination that a chilled liquid temperature isgreater than a setpoint temperature plus an offset temperature.
 4. Themethod of claim 3 wherein the step of starting the predetermined numberof inverters of the plurality of inverters includes the step of startingthe predetermined number of inverters at a predetermined frequency. 5.The method of claim 4 wherein the predetermined frequency is a frequencybetween about 15 Hz and about 75 Hz.
 6. The method of claim 5 whereinthe predetermined frequency is about 40 Hz.
 7. The method of claim 5wherein: the setpoint temperature is a temperature between about 15° F.and about 55° F.; and the offset temperature is a temperature betweenabout ±1° F. and about ±5° F.
 8. The method of claim 1 wherein the stepof adjusting an operating configuration of the plurality of inverters toincrease the output capacity of a multiple compressor chiller systemincludes the step of determining whether any inverter of the pluralityof inverters is inactive.
 9. The method of claim 8 wherein the step ofadjusting an operating configuration of the plurality of inverters toincrease the output capacity of a multiple compressor chiller systemfurther includes the step of determining if an inverter operatingfrequency for the plurality of inverters is greater than a stopfrequency plus a predetermined offset frequency.
 10. The method of claim9 wherein the step of adjusting an operating configuration of theplurality of inverters to increase the output capacity of a multiplecompressor chiller system further includes the step of starting anadditional inverter of the plurality of inverters in response to adetermination that any inverter of the plurality of inverters isinactive and a determination that an inverter operating frequency forthe plurality of inverters is greater than a stop frequency plus apredetermined offset frequency.
 11. The method of claim 10 wherein thestep of starting an additional inverter of the plurality of invertersincludes the step of configuring operating inverters of the plurality ofinverters to operate at a predetermined frequency.
 12. The method ofclaim 11 wherein the predetermined frequency is the inverter operatingfrequency multiplied by a ratio of a number of operating invertersdivided by the number of operating inverters plus one.
 13. The method ofclaim 9 wherein the stop frequency is a minimum inverter frequencymultiplied by a ratio of a number of operating inverters plus onedivided by the number of operating inverters.
 14. The method of claim 12wherein: the minimum inverter frequency is a frequency between about 15Hz and about 75 Hz; and the predetermined offset frequency is afrequency between about 0 Hz and about 50° Hz.
 15. The method of claim 8wherein the step of adjusting an operating configuration of theplurality of inverters to increase the output capacity of a multiplecompressor chiller system further includes the step of determiningwhether an inverter operating frequency is less than a maximum inverterfrequency.
 16. The method of claim 15 wherein the step of adjusting anoperating configuration of the plurality of inverters to increase theoutput capacity of a multiple compressor chiller system further includesthe step of increasing the inverter operating frequency by apredetermined frequency amount in response to a determination that noinverter of the plurality of inverters is inactive and a determinationthat an inverter operating frequency is less than a maximum inverterfrequency.
 17. The method of claim 16 wherein the predeterminedfrequency amount is a frequency between about 0.1 Hz and about 25 Hz.18. The method of claim 9 wherein the step of adjusting an operatingconfiguration of the plurality of inverters to increase the outputcapacity of a multiple compressor chiller system further includes thestep of increasing the inverter operating frequency by a predeterminedfrequency amount in response to a determination that any inverter of theplurality of inverters is inactive and a determination that an inverteroperating frequency of the plurality of inverters is not greater than astop frequency plus a predetermined offset frequency.
 19. The method ofclaim 1 wherein the step of adjusting an operating configuration of theplurality of inverters to decrease the output capacity of a multiplecompressor chiller system includes the step of determining whether onlyone inverter of the plurality of inverters is operating.
 20. The methodof claim 19 wherein the step of adjusting an operating configuration ofthe plurality of inverters to decrease the output capacity of a multiplecompressor chiller system further includes the step of determining if aninverter operating frequency is greater than a minimum inverterfrequency.
 21. The method of claim 20 wherein the step of adjusting anoperating configuration of the plurality of inverters to decrease theoutput capacity of a multiple compressor chiller system further includesthe step of determining whether a chilled liquid temperature is lessthan a setpoint temperature minus an offset temperature.
 22. The methodof claim 21 wherein the step of adjusting an operating configuration ofthe plurality of inverters to decrease the output capacity of a multiplecompressor chiller system further includes the step of stopping the onlyone operating inverter of the plurality of inverters in response to adetermination that only one inverter of the plurality of inverters isoperating, a determination that an inverter operating frequency is notgreater than a minimum inverter frequency, and a determination that achilled liquid temperature is less than a setpoint temperature minus anoffset temperature.
 23. The method of claim 22 wherein: the minimuminverter frequency is a frequency between about 15 Hz and about 75 Hz;the setpoint temperature is a temperature between about 15° F. and about55° F.; and the offset temperature is a temperature between about ±1° F.and about ±5° F.
 24. The method of claim 19 wherein the step ofadjusting an operating configuration of the plurality of inverters todecrease the output capacity of a multiple compressor chiller systemfurther includes the step of determining if an inverter operatingfrequency is substantially equal to a minimum inverter frequency. 25.The method of claim 24 wherein the step of adjusting an operatingconfiguration of the plurality of inverters to decrease the outputcapacity of a multiple compressor chiller system further includes thestep of stopping an operating inverter of the plurality of inverters inresponse to a determination that more than one inverter of the pluralityof inverters is operating and a determination that an inverter operatingfrequency is equal to a minimum inverter frequency.
 26. The method ofclaim 25 wherein the step of stopping an operating inverter of theplurality of inverters includes the step of configuring the remainingoperating inverters of the plurality of inverters to operate at apredetermined frequency.
 27. The method of claim 26 wherein thepredetermined frequency is a minimum inverter frequency multiplied by aratio of the remaining operating inverters plus one divided by theremaining operating inverters.
 28. The method of claim 27 wherein theminimum inverter frequency is a frequency between about 15 Hz and about75 Hz.
 29. The method of claim 20 wherein the step of adjusting anoperating configuration of the plurality of inverters to decrease theoutput capacity of a multiple compressor chiller system further includesthe step of decreasing the inverter operating frequency by apredetermined frequency amount in response to a determination that onlyone inverter of the plurality of inverters is operating and adetermination that an inverter operating frequency is greater than aminimum inverter frequency.
 30. The method of claim 29 wherein thepredetermined frequency amount is a frequency between about 0.1 Hz andabout 25 Hz and the inverter minimum frequency is a frequency betweenabout 15 Hz and about 75 Hz.
 31. The method of claim 24 wherein the stepof adjusting an operating configuration of the plurality of inverters todecrease the output capacity of a multiple compressor chiller systemfurther includes the step of decreasing the inverter operating frequencyby a predetermined frequency amount in response to a determination thatmore than one inverter of the plurality of inverters is operating and adetermination that an inverter operating frequency is not equal to aminimum inverter frequency.
 32. A multiple compressor chiller systemcomprising: a plurality of compressors, each compressor of the pluralityof compressors being driven by a corresponding motor, the plurality ofcompressors being incorporated into at least one refrigerant circuit,each refrigerant circuit comprising at least one compressor of theplurality of compressors, a condenser arrangement and an evaporatorarrangement connected in a closed refrigerant loop; a variable speeddrive to power the corresponding motors of the plurality of compressors,the variable speed drive comprising a converter stage, a DC link stageand an inverter stage, the inverter stage having a plurality ofinverters each electrically connected in parallel to the DC link stageand each powering a corresponding motor of the plurality of compressors;and a control panel to control the variable speed drive to generate apreselected system capacity from the plurality of compressors, thecontrol panel being configured to determine a number of inverters of theplurality of inverters to operate in the variable speed drive andconfigured to determine an operating frequency for the number ofoperating inverters of the plurality of inverters in the variable speeddrive to generate the preselected system capacity from the plurality ofcompressors.
 33. The multiple compressor chiller system of claim 32wherein the control panel comprises means for enabling a predeterminednumber of inverters of the plurality of inverters in response to a startcontrol signal and satisfying at least one predetermined start criteria.34. The multiple compressor chiller system of claim 33 wherein the atleast one predetermined start criteria comprises at least one of: theplurality of inverters being inactive; and a chilled liquid temperaturebeing greater than a setpoint temperature plus an offset temperature.35. The multiple compressor chiller system of claim 34 wherein the meansfor enabling a predetermined number of inverters includes means foroperating the predetermined number of inverters at a predeterminedfrequency.
 36. The multiple compressor chiller system of claim 32wherein the control panel comprises means for adjusting the operatingfrequency of the plurality of inverters of the variable speed drive inresponse to satisfying at least one predetermined capacity criteria,means for enabling a non-operating inverter of the plurality ofinverters in response to satisfying at least one predetermined enablingcriteria, and means for disabling an operating inverter of the pluralityof inverters in response to satisfying at least one predetermineddisabling criteria.
 37. The multiple compressor chiller system of claim36 wherein: the means for adjusting the operating frequency of theplurality of inverters of the variable speed drive comprises means forincreasing the operating frequency of the plurality of inverters by apredetermined frequency amount; and the at least one predeterminedcapacity criteria comprises at least one of: an increase capacitycontrol signal; any inverter of the plurality of inverters beingdisabled; an inverter operating frequency being less than a maximuminverter frequency; all inverters of the plurality of inverters beingenabled; and an inverter operating frequency for the plurality ofinverters being less than a stop frequency plus a predetermined offsetfrequency.
 38. The multiple compressor chiller system of claim 37wherein the predetermined frequency amount is a frequency between about0.1 Hz and about 25 Hz.
 39. The multiple compressor chiller system ofclaim 36 wherein: the means for adjusting the operating frequency of theplurality of inverters of the variable speed drive comprises means fordecreasing the operating frequency of the plurality of inverters by apredetermined frequency amount; and the at least one predeterminedcapacity criteria comprises at least one of: a decrease capacity controlsignal; more than one inverter of the plurality of inverters beingenabled; an inverter operating frequency being not equal to a minimuminverter frequency; only one inverter of the plurality of invertersbeing enabled; and an inverter operating frequency being greater than aminimum inverter frequency.
 40. The multiple compressor chiller systemof claim 39 wherein the predetermined frequency amount is a frequencybetween about 0.1 Hz and about 25 Hz.
 41. The multiple compressorchiller system of claim 36 wherein the at least one predeterminedenabling criteria comprises at least one of: an increase capacitycontrol signal; any inverter of the plurality of inverters beingdisabled; and an inverter operating frequency for the plurality ofinverters being greater than a stop frequency plus a predeterminedoffset frequency.
 42. The multiple compressor chiller system of claim 36wherein the at least one predetermined disabling criteria comprises atleast one of: a decrease capacity control signal; more than one inverterof the plurality of inverters being enabled; an inverter operatingfrequency being equal to a minimum inverter frequency; only one inverterof the plurality of inverters being enabled; an inverter operatingfrequency being less than a minimum inverter frequency; and a chilledliquid temperature being less than a setpoint temperature minus anoffset temperature.
 43. A capacity control method for a multiplecompressor chiller system, the method comprising the steps of: providinga variable speed drive having a plurality of inverters, wherein eachinverter is configured to power a corresponding compressor motor of amultiple compressor chiller system at a preselected output frequency;monitoring at least one operating condition of a multiple compressorchiller system; determining whether to increase capacity in the multiplecompressor chiller system in response to the at least one monitoredoperating condition; configuring the plurality of inverters to generateincreased capacity in the multiple compressor chiller system in responseto a determination to increase capacity, wherein the step of configuringthe plurality of inverters to generate increased capacity includes:determining whether to enable an additional inverter of the plurality ofinverters in order to start an additional compressor motor of themultiple compressor chiller system; enabling an additional inverter ofthe plurality of inverters in response to a determination to enable anadditional inverter; and adjusting the preselected output frequency ofeach operating inverter of the plurality of inverters; determiningwhether to decrease capacity in the multiple compressor chiller systemin response to the at least one monitored operating condition; andconfiguring the plurality of inverters to generate decreased capacity inthe multiple compressor chiller system in response to a determination todecrease capacity, wherein the step of configuring the plurality ofinverters to generate decreased capacity includes: determining whetherto disable an operating inverter of the plurality of inverters in orderto stop a compressor motor of the multiple compressor chiller system;disabling an operating inverter of the plurality of inverters inresponse to a determination to disable an operating inverter; anddecreasing the preselected output frequency of each operating inverterof the plurality of inverters.
 44. The capacity control method of claim43 further comprising the steps of: determining whether to start apredetermined number of inverters of the plurality of inverters inresponse to the at least one monitored operating condition; and startingthe predetermined number of inverters of the plurality of inverters inresponse to a determination to start the predetermined number ofcompressors.
 45. The capacity control method of claim 44 wherein thestep of starting the predetermined number of inverters of the pluralityof inverters includes the steps of: determining whether the plurality ofinverters are disabled; determining whether a chilled liquid temperatureis greater than a setpoint temperature plus an offset temperature; anddetermining the predetermined number of inverters of the plurality ofinverters to start in response to a determination that the plurality ofinverters are disabled and a determination that a chilled liquidtemperature is greater than a setpoint temperature plus an offsettemperature.
 46. The capacity control method of claim 45 wherein thestep of starting the predetermined number of inverters of the pluralityof inverters includes the step of operating the predetermined number ofinverters at a predetermined frequency.
 47. The capacity control methodof claim 43 wherein the step of determining whether to enable anadditional inverter of the plurality of inverters includes the steps of:determining whether any inverter of the plurality of inverters isdisabled; and determining whether an inverter operating frequency forthe plurality of inverters is greater than a stop frequency plus apredetermined offset frequency.
 48. The capacity control method of claim43 wherein the step of adjusting the preselected output frequency ofeach operating inverter of the plurality of inverters includes the stepsof: determining whether any inverter of the plurality of inverters isdisabled; determining whether an inverter operating frequency of theplurality of inverters is less than a stop frequency plus apredetermined offset frequency; and increasing the inverter operatingfrequency by a predetermined frequency amount in response to adetermination that any inverter of the plurality of inverters isdisabled and a determination that an inverter operating frequency of theplurality of inverters is less than a stop frequency plus apredetermined offset frequency.
 49. The capacity control method of claim43 wherein the step of determining whether to disable an operatinginverter of the plurality of inverters includes the steps of:determining whether only one inverter of the plurality of inverters isenabled; determining whether an inverter operating frequency is lessthan a minimum inverter frequency; and determining whether a chilledliquid temperature is less than a setpoint temperature minus an offsettemperature.
 50. The capacity control method of claim 43 wherein thestep of decreasing the preselected output frequency of each operatinginverter of the plurality of inverters includes the steps of:determining whether only one inverter of the plurality of inverters isenabled; determining whether an inverter operating frequency is greaterthan a minimum inverter frequency; and decreasing the inverter operatingfrequency by a predetermined frequency amount in response to adetermination that only one inverter of the plurality of inverters isoperating and a determination that an inverter operating frequency isgreater than a minimum inverter frequency.
 51. The capacity controlmethod of claim 43 wherein the step of adjusting the preselected outputfrequency of each operating inverter of the plurality of invertersincludes the steps of: determining whether all inverters of theplurality of inverters are enabled; determining whether an inverteroperating frequency is less than a maximum inverter frequency; andincreasing the inverter operating frequency by a predetermined frequencyamount in response to a determination that all inverters of theplurality of inverters are enabled and a determination that an inverteroperating frequency is less than a maximum inverter frequency.
 52. Thecapacity control method of claim 43 wherein the step of determiningwhether to disable an operating inverter of the plurality of invertersincludes the steps of: determining whether more than one inverter of theplurality of inverters is enabled; and determining whether an inverteroperating frequency is equal to a minimum inverter frequency.
 53. Thecapacity control method of claim 43 wherein the step of decreasing thepreselected output frequency of each operating inverter of the pluralityof inverters includes the steps of: determining whether more than oneinverter of the plurality of inverters is enabled; determining whetheran inverter operating frequency is not equal to a minimum inverterfrequency; and decreasing the inverter operating frequency by apredetermined frequency amount in response to a determination that morethan one inverter of the plurality of inverters is enabled and adetermination that an inverter operating frequency is not equal to aminimum inverter frequency.